Waste paper that has been printed with ink or other printing material or other types of contaminated (non-printed) wastepaper can be recycled by repulping the paper and removing the ink and contaminants therefrom in order to produce acceptable pulp fiber for renewed papermaking. Conventional wastepaper processing systems have employed a finely slotted screen for separating contaminants from the repulped fiber in suspension. However, conventional screens (even with very fine slots of the order of 0.006") cannot effectively remove ink particles which due to their small size and flexibility tend to slip through even the fine slotting. Present technology has also employed agglomeration techniques for producing some ink agglomerates of a size that can be removed through screening. However, it is found that other fine ink agglomerates are still not removed in sufficient quantities using the conventional screening methods.
Various other proposals have been made for ink removal from recycled pulp. U.S. Pat. No. 4,215,447 to Gartland discloses one technique for washing high consistency fiber (up to 7%) stock by sweeping the ink in a divergent flow from the fiber. The washing apparatus includes a cylinder with a mesh screen and impeller bars rotated about a vertical axis within the screen. A "grey stock" containing ink particles and repulped fiber in suspension is fed into the bottom of the cylinder, while washing water is added through the top into an apertured inner chamber at the center of the cylinder. As the rising "grey stock" makes contact with the radially moving washing water under the stirring action of the rotating blades, the ink particles are swept radially outward through the mesh, while the fiber flows in suspension to an outlet at the top of the apparatus.
Another type of approach is shown in U.S. Pat. No. 4,397,741 to Miller for separating particles by flotation from a forced vortex. An upright cylindrical vessel has a tangential inlet at its upper end for feeding in the pulp slurry and an outlet at the lower end for the output cleaned fiber. The slurry is fed in a flow that creates a forced vortex in the vessel. A portion of the vessel wall is porous, and air is injected into the fluid layer moving along the vessel wall. The air bubbles become carriers for particle aggregates which float to the top of the vessel, so that cleaned pulp can be discharged at the bottom of the apparatus. U.S. Pat. No. 4,997,549 to Atwood shows another flotation apparatus that includes a froth washing tube within the vessel for introducing wash water under pressure.
U.S. Pat. No. 4,157,952 to Krofta shows another apparatus used in a flotation deinking process that has water injector nozzles and air inlets positioned to aerate the slurry and feed the slurry tangentially in a swirling rotation within the flotation tank. As the fluid spirals towards the center of the tank, the froth or foam bubbles carrying the ink particles float to the surface and are removed by a vacuum outlet.
Other patents showing flotation-type cleaners include U.S. Pat. Nos. 3,557,956, 3,635,788, and 3,912,579 to Braun; U.S. Pat. Nos. 4,013,505 and 4,147,616 to Balcar; U.S. Pat. No. 4,726,880 to Smith; U.S. Pat. Nos. 5,069,751 and 5,131,980 to Chamblee and U.S. Pat. No. 5,173,177 to Greenwood.
However, the prior types of flotation cleaners require large, expensive systems and equipment, and high concentrations of expensive agglomeration and flotation chemicals in order to obtain satisfactorily levels of ink removal. It would be desirable to have a pulp cleaning system requiring less costly or complex equipment and which can achieve high levels of ink and contaminant removal, particularly fine ink particles, without using high dosages of chemicals.
Accordingly, it is a broad object of the invention to provide a pulp cleaning apparatus and related processes for the effective removal of ink particles and contaminants from paper pulp.
Another object of the invention is to provide a low cost apparatus that effectively and efficiently removes ink particles and contaminants without using large amounts of expensive agglomeration or flotation chemicals.
A more specific object of the invention is to improve the overall yield of the deinking process, as well as improve the removal of fine ink particles. It is particularly desired to increase the recovered pulp's cleanliness and brightness by removing high levels of the fine ink particles which reduce brightness, and thereby achieve a reduction in the requirement for expensive bleaching chemicals.